PCI: spear: Add PCIe driver for ST Microelectronics SPEAr13xx

ARM based ST Microelectronics's SPEAr1310 and SPEAr1340 SOCs have onchip
designware PCIe controller. To make that usable, this patch adds a wrapper
driver based on existing designware driver.

Adds bindings for this new driver and update MAINTAINERS as well.

Cc: linux-pci@vger.kernel.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Mohit Kumar <mohit.kumar@st.com>
[viresh: fixed logs/cclist/checkpatch warnings, broken into smaller patches]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>